The present invention relates to an optical amplifier having an active fiber doped with ions of elements from the group of rare earths, and an optical amplifier arrangement having a number of cascaded optical amplifier stages each having an active fiber doped with ions of elements from the group of rare earths.
In optical transmission systems for transmitting optical signals at high channel data rates, signal distortions occur as a result of nonlinear effects in the optical fibers or transmission fibers. In particular, the nonlinear effect of cross phase modulation (XPM) can distinctly impair the transmission properties in optical transmission systems with many channels, particularly wavelength division multiplexing (WDM) channels. The active fibers of the optical amplifiers are short compared with the transmission fibers of the optical path sections and thus, they contribute only a fraction of the total length of the optical transmission system. As such, only a small portion of the cross phase modulation contributions occurring in the entire optical transmission system should be attributable to them. However, due to the high signal levels in the optical amplifiers or optical amplifier arrangements and in their active fibers, small mode field diameters can nevertheless lead to an appreciable system impairment due to the cross phase modulation contributions generated there. In this respect, see, M. Shtaif, M. Eiselt: xe2x80x9cNonlinearities in Erbium-doped Fiber Amplifiersxe2x80x9d, Conference on Optical Amplifiers and their Applications (1999), June 9-11, Nara, Japan, FC2, pp. 270-272.
What is critical for the system impairments caused by the cross phase modulation, in particular at high signal levels, is the fiber length in which two WDM channels experience a slip of at least one bit length. This length is referred to as xe2x80x9cwalk-off length,xe2x80x9d for example, in M. Shtaif, M. Eiselt: xe2x80x9cNonlinearities in Erbium-doped Fiber Amplifiersxe2x80x9d, Conference on Optical Amplifiers and their Applications (1999), June 9-11, Nara, Japan, FC2, pp. 270-272. The walk-off length and also the influence of the cross phase modulation decreases with rising dispersion.
In known optical transmission systems, in particular WDM transmission systems, the system impairment due to the cross phase modulation in the optical amplifiers or amplifier arrangements is manifested only to a slight extent as a result of the low signal levels or large channel spacings used in the transmission of optical signals. Measures for reducing the cross phase modulation in the optical amplifiers are not provided, therefore, in currently known or commercially available optical amplifiers or amplifier arrangements. However, the cross phase modulation contributions, and thus system impairments, brought about by the optical amplifiers will increase in future optical transmission systems due to the rising number of WDM channels required for transmission and the reduced channel spacings as a result of this.
Furthermore, optical fiber amplifiers or amplifier arrangements, in particular erbium-doped fiber amplifiers, developed specifically for the utilization of the L transmission band have longer active fibers than the optical amplifiers designed for the C transmission band. For the aforementioned reasons, reducing the cross phase modulation in optical amplifiers, in particular for future optical transmission systems, is accorded immense importance.
An object of the present invention, therefore, is to reduce the nonlinear effect of cross phase modulation which forms in the active fiber of an optical amplifier or optical amplifier arrangement. Such object is achieved on the basis of an optical amplifier in accordance with the teachings of the present invention.
An advantage of the present invention""s optical amplifier having an active fiber doped with ions of elements from the group of rare earths can be seen in the fact that the active fiber has at least one active fiber section for reducing the cross phase modulation of the optical amplifier, the active fiber section being allocated a dispersion coefficient having a high magnitude, in which the optical transmission signal to be amplified assumes a high signal level. In the amplifier according to the present invention, the active fiber is advantageously designed in such a way that, in the active fiber sections in which high signal levels occur, the active fiber in each case has a high dispersion coefficient which, in turn, leads to a major slip of the channels. Due to the major slip between the channels or WDM channels, the nonlinear effect of cross phase modulation is considerably reduced in the active fiber. However, to ensure that the high dispersion coefficients of the active fibers do not, for their part, lead to signal distortions within the optical transmission signal, the active fiber section of the active fiber in which high signal levels occur (usually the last part of the active fiber) is allocated a dispersion coefficient having a high magnitude; i.e., a high positive or negative dispersion coefficient.
The lengths of the individual active fiber sections are advantageously calculated taking account of the dispersion coefficients present in the active fiber sections in such a way that the total dispersion of the amplifier assumes a minimum value or the optical amplifier has a total dispersion of at least virtually zero.
Additional features and advantages of the present invention are described in, and will be apparent from, the following detailed description of the invention.